NMR is a well established technique for chemical analysis. It is based upon the fact that atomic nuclei have magnetic moments and angular momentum or spin. When the nuclei are placed in a strong, uniform, and constant magnetic field they tend to line up with the field to give a net magnetization. When the magnetization is reoriented, it will precess about the direction of the strong magnetic field at a frequency called the Larmor frequency. This frequency is proportional to the magnetic field strength. The frequency will depend to a smaller extent upon the position and environment of each nuclei in a molecule, making NMR spectroscopy an extremely powerful technique for the determination of molecular structure. Typical NMR nuclei include .sup.1 H (protons), .sup.13 C (carbon), .sup.19 F (fluorine) and .sup.31 P (phosphorous).
Normally the sample material to be analyzed in an NMR instrument is dissolved by a solvent and placed into a sample tube. The sample tube is inserted into a probe which contains a transmitter coil that is excited with radio frequency energy that is used to tip the nuclear magnetization away from the direction of the magnet field and to cause the start of the precession. The precessing magnetization induces a voltage in the receiver coil of the probe which is coupled to an amplifier and detector circuits. In most modern NMR spectrometer equipment, the receiver and transmitter are turned on and off very quickly so that the receiver is off when the transmitter is on, and vice versa so that a single coil is used which is known as the transmit/receive coil or the probe coil.
The sensitivity of an NMR spectrometer depends upon many factors including the magnetic field strength, the quality factor Q of the receiver coil used to pick up the precessing magnetization and the degree of coupling (characterized by the filling factor) between the probe coil and the NMR sample and thermal noise contributed by the sample material. To achieve the highest magnetic fields modem spectrometers use superconducting magnets operating at field strengths as high as 17 tesla. For proton resonance, the typical Q of the receiver coil is in the range of 400-500 for coils using normal conductors. Recently spectrometers have been built using receiver coils made of High Temperature Superconductor (HTS) materials that have Q's approximately 100 times higher, i.e. up to 50,000. With coil Q's in this range the loading by the sample becomes significant for somewhat lossy samples reducing the overall Q and sensitivity of the spectrometer. The purpose of this invention is to decrease the loss of Q due to the sample material without substantially reducing the filling factor and thereby enabling an increase in sensitivity.